Heretofore, a shovel-type excavator 150 (FIG. 5) is provided with a propelling mechanism 153 having a pair of caterpillar tracks 152 driven by a propelling means 151. A rotatable base assembly 155 mounted on the propelling mechanism 153 affords support for an articulated bucket arm 154 which may be inclined at any desired angle by a hydraulic cylinder, operator's cab, engine for providing operating power to a hydraulic pump which supplies hydraulic fluid under pressure to the hydraulic cylinder, and bulldozer.
In order to do the excavation work and the work of loading the earth into transport vehicles by means of this prior art shovel-type excavator 150, the ingress and egress of hydraulic fluid from hydraulic cylinders allotted respectively for the propelling mechanism 153 and the articulated bucket arm 154 are effected from a control panel provided in the operator's cab.
Various types of propelling means 151 for the propelling mechanism 153 are known, including the one disclosed in Japanese Laid Open Utility Model Application No. 4-1710. FIG. 6 is a view of a propelling means 151 in accordance with the prior art.
The propelling means 151 shown in FIG. 6 includes a hydraulic motor 160 and a speed reducer 161. The latter takes the form of a planetary gear set, in which the outer portions of planetary gears are in mesh engagement with an internal ring gear 166. The internal ring gear 166 constitutes the output side of the speed reducer 161 and is rotatably mounted on the casing 164 of the hydraulic motor 160. The caterpillar track 152 cooperates with a sprocket wheel 156 fixed on the internal ring gear 166.
The rotation of the output shaft 162 of the hydraulic motor 160 will effect a rotation of a second sun gear 168 through a first sun gear 163 and the planetary gears 167. A carrier 165 secured to the hydraulic motor 160 rotatably carries carrier gears 169, through which the rotating second sun gear 168 will effect a rotation of the internal ring gear 166, which in turn will drive the caterpillar track through the sprocket wheel 156.
The output shaft 162 of the hydraulic motor 160 is rotatably supported by the casing 164 and carries a cylinder block 170 having a plurality of pressure chambers, in each of which an axially movable piston 171 is slidably mounted. The end of the piston 171 is adapted to engage a bearing plate 172 seated on the casing 164 at an oblique angle to the axis thereof. When a pressure chamber having an opening disposed in the immediate vicinity of the bearing plate 172 is replenished with hydraulic oil, the piston 171 accommodated therein is shifted toward extend position. Consequently, the end of this piston 171 bears upon the bearing plate 172 and causes the output shaft 162 to rotate.
A spring brake 180 interposed between the casing 164 and the cylinder block 170 includes a spring 174. When the hydraulic oil is displaced from the hydraulic motor 160, the cylinder block 170 which has been rotating is yieldingly biased by the spring 174 to a standstill. A counterbalance valve 173 serves to keep the pressure applied to the ingressive hydraulic oil higher than the pressure applied to the egressing hydraulic oil.
It is a matter of course that the propelling mechanism of a construction machine should not have a portion disposed sideways away from the outer edge of the caterpillar track. If the propelling mechanism has such a portion, it may well be that this portion will strike against an obstacle such as stones, tiles, pebbles or scrap wood and will be damaged thereby.
There is no teaching in the prior art that a compact arrangement requiring less dimension in the transverse direction may be attained with respect to the components of the propelling mechanism such as the hydraulic motor and the speed reducer, without detriment to the productivity in manufacturing these components.
It,is accordingly a principal object of the present invention to provide a crawler drive unit in which a compact arrangement requiring less dimension not only in the transverse direction but also in the longitudinal direction is attained with respect to the components of the propelling mechanism.